Vacuum tube comparison test method and system



Aug. 9, 1949. J/J. DE: MUTH VACUUM TUBE COMPARISON TEST METHOD AND SYSTEM 2 Sheets-Sheet l Filed'April 17, 1948 mi. SQMIN* Illul zNr/ENToR. cfa/12V JJDE Mum BY Arma/wmf Aug. 9, 1949. J. J. DE MUTH VCUM TUBE COMPARISON TEST METHOD AND SYSTEM Filed April 17, 1948 2 Sheets-Sheet 2 AMPL/F/ER fas . mwk;

JNVENTOR. JaHN Jl: M1111 BWK/@nd ATTRZVEX Patented Aug. 9, 1949 VACUUM TUBE COMPARISON 'TEST METHOD AND SYSTEM I'ohn J. De Muth, Burbank, Calif., vassignor "to Radio Corporation of America, a corporatonol Delaware Application April 17, 1948, "Serial No. 21,697v

inventionrelates to electrical circuits, and particularly to a test circuitfor selecting pairs of electron vacuum tubes having substantially identical or .parallel characteristics.

In systems using variable .gain amplifiers, and particularly in compressiomexpander, and volume control circuits, wherein either an independcnt signal or the signalY being amplified controls the 'amplifier A'gain-a`r1d thus-the volume level of the signal being .transmittedthe control is usually obtained by varying the grid bias on a. pair of variable mu tubes connected npush-pull. The pair of tubes operatingl inthe "variable gain stage should have characteristics as nearly identical or parallel as possible to prevent the introduction of spurious currents duringthe variations in gain of the tubes. "Ihat.is, should one tube not produce the same variationin output current as the 'other for the samer `grid variation, then a differ- 'ential current is introduced, which is commonly known as a thuinn The matching of the pushpull tubes and the balancing of the push-pull circuit eliminates the Yspuriouseurrents. A variable gain circuit of the above type i'sdisclosed and claimed iin-Singer U. VfyPat'ent fNo. 2,255,683 of September 9, 194:1.v

Several vsystems for determining the static and dynamic baiance o'fa push-pull'circuit and the tubes used thereinhave been suggested, such as 'disclosed and claline'dn Bayless U. S. Patent No.

2,385,186 ofsSeptember. L8, 31945, this circuit being one to determine the dynamic balance of `the tubes and circuits.V Othergsystems for determining the similarity of the characteristics of pushpull tubes at 'one ormore settings of the grid bias are sho'wn in Lavoie 'U'.JS. Patent No. 1,948,303 of February 20, 1934, and Miessner U. S. Patent No.v 1,946,092 of February v6, 1934. The systems shown in the above patents are ycries wherein the actual 4system in which the tubes are to be used may be tested with the tubes in the circuit. The present invention,` however, is directed primarily to a system whereby pairs of matching tubes may be selected from -|a group vof tubes, these pairs then being-suitable Afor use ina system such as 'shown in the 4above mentioned Sin-ger circuit.

The inventionprovidesa rapid method of selecting and determining thedynamic 'characteristi'cs over a wide range of `negative 'grid bias, and

also provides 'a `statici-,est over a vshort section of the grid voltage-plate current characteristics. The inventiony `also Iincludes lfacircuit for making -a quick eld test of a pair of tubes when inserted in a circuit in which they are to be used, in the event that oneor the. other drifted 'Since they 15 Claims. (Cl. 315-4379)- were selected as a matched pair. ""I'hislatter circuit also vpermits the balance controlof the amplier to be adjusted -to an opti-mum .position for dynamic operation. A clamping circuitis embodied in the main testing-.system whereby only negative voltages may` Abe vderived Virom `a normal alternating current. The y multiple section switches connect the inputof each tube in parallel with every other tube.A andthe outputs of t each selected pair of tubes push-pull. Thus,

the variations in transmission characteristics are quickly indicated.

The principal object of the invention, therefore,

is to facilitate the ,matching ofl electronvacuum tubes.v

f vide an improved test method and system for selecting electron tube pairs having similar or parallel characteristics. Y

Although the novel features which are believed to be characteristic of 4this invention will be pointed out with Aparticularity'in the appended claims, the manner of its organization and the mode -of its operation will beibetter understood Vbyreferring to theiollowing description read -in conjunction with the accompanying drawings, forming a. part hereof. in which-z Fig.l l is a schematic circuit of fa test system embodying the invention.

Fig. 2 is a graph showing `the operation of the clamp circuit of the test system of Fig. l.

Fig. '3 is a graph showing `several tube characteristics, and

Fig. 4 is a Sclfiem'atic circuit drawing of a iield test system. l

Referring now to the drawings. and particu- Ylarly( referring to Fig? l.y an oscillator v5, which may be tuned to providean outputcurrent'having a frequency in the neighborhood ofseventy cycles, is connected through a Tepadj to a gang switch 'l hav-ing left-hand -contacts l0. and Il and right-hand contacts 'i3'.- H, and-`|5. The next element in the system is athree-resistor-attenuator pad I8 which -is connected inthe circuit when the switch 1 is thrown tothe right, and is eliminated-trom the circuit whentheswitch 'l is thrown to the left.

The output of oscillator is impressed on a v l transformer I 9, the secondary of which has a terminating resistor 26 connected across it, and which is then capacity coupled over a condenser 22 and conductor 23 to the swingers 24 and 25 of the first or upper sections of gang switch units 26 and 21, respectively. The other terminal of the secondary of transformer I9 is connected, over a cathode resistor 36 having a value comparable to that used in the usual commercial variable gain amplifier and conductor 28 to the swingers 3l and 32 of the second or middle sections of switch units 26 and 21. A resistor 34 is in shunt across the secondary of transformer I9 and simulates the timing resistor of a normal variable gain amplifier. This resistor is also one of the elements of the clamp circuit including the condenser 22 and a diode tube 35 connected in shunt across the secondary circuit over conductors 36and 31 when the switch 1 is thrown to contact II. Y

The right-hand portion of the circuit includes a balancing resistor and condenser network 39 composed of resistor-s 4 6, 4I, 42, 43, and 44, together with a condenser 41, condensers 46 and 48 being blocking condensers. The output of this network is impressed on a transformer 56,

Vthe secondary of which is connectedV to an amplier 5I feeding a meter 52, the latter being any standard volume indicator.

The present circuit has Ibeen designed to compare twentyone tubes by the use of switch units 26 and 21 operating in conjunction with multiplier switch units 54 and 55. This is accomplished by Aconnecting swinger 24 `of unit 26 to a swinger 56 of unit`54 when the former is moved to its last contact, and by connecting swinger 25 of unit 271 to a swinger 51 of unit 55 when swinger 25 is moved to its last contact. The same connections are made for the second sections of the switch units, that is, swinger 3I is connected with swinger 59, and swinger 32 is connected with swinger 69. Each switch unit 26, 21, 54, and 55 also has a third or lower section ganged with the first and second sections, as indicated by the broken lines, a swinger 62 of the third section of unit 26 connecting with swinger 63 when swinger 62 is on its last contact, and swinger 64 connecting with swinger 65 when swinger 64 is on its last contact. As mentioned above, the rst two sections of the ganged switch units transfer the output of transformer I9 to selected tubes in parallel, while the third section-s of the respective units transfer the output of the tubes to network 39 in push-pull.

For purposes of illustration, only three tubes 68, 69, and 16 are shown under test for the sake of clarity of explanation, it being understood that the other eighteen tubes are connected to the eleven contacts of switch units 54 and 55 in a similar manner and to the remaining seven contacts of the switch units 26 and 21 in a similar manner. Thus, the tubes connected to the eleven contacts of units 54 and 55 are connectable into the system when the swingers of units 26 and 21 are on their last contacts.

To trace out the circuit for connecting tubes 66, 69, and 16 into the system, it will be noted that the anode of tube 16 is connected over conductors 12, 13, and 14 to the first contacts of the third sections of switch units 26 and 21. Swinger 62 is now connected to one terminal of network 39 over conductor 15, and swinger 64 is connected to the other terminal of network 39. Thus, the anode of tube 16 may be connected to either terminal of the output push-pull network, depending upon the terminal to which the tube with which it is being compared is connected. Likewise, the anode of tube 69 may be connected to either terminal of the network over the second contacts of the third section of switch units 26 and 21, while the anode of tube 68 is similarly wired over the third contacts of these switches. Thus, with swinger 64 on its rst contact, and swinger 62 on its second contact, the outputs of tube 69 and tube 16 are connected in push-pull to the network 39 and any diiference in these outputs will be `indicated on meter 52. If the swinger 62 is thrown to its third contact position, then tube 66 will be connected in the system and its output can be compared with that of tube 19. If swinger 64 is swung to its second contact, and swinger 62 to its third contact, then the outputs of tubes 68 and 69 may be compared. Thus, the third sections of the switch units permit the outputs of any pair of tubes to be connected in 'to the rst contact of the second or middle sections of switch units 26 and 21, while the grid of tube 16 is connected over conductors 82, 83, and 64 to the first contact of the first or upper sections of switch units 26 and 21. Similar connections are provided to connect the grids and cathodes of tubes 69 and 68 to the second and third contacts, respectively, of the rst and second sections. Now, since the three sections of the respective switch units are ganged together, then, when the anode or output of a tube is connected to one or the other of the input terminals to network 39, its input is connected across the `secondary of transformer I9. Thus, the inputs of any pair of tubes selected are in parallel across transformer I9, and each tube receives the same signal input. So, if the characteristics of the tubes are identical, then there will be no reading at meter 52.

The input circuits are over swingers 24 and 25, which connect the control grid of tube 16 over conductor 23 to one terminal of the secondary of input transformer I9, and swingers 3| and 32 which connect the cathode of Ytube 16 over conductor 28 and resistor 36 to the other terminal of the secondary of transformer I9. These last mentioned swingers, in cooperation with swingers 56, 51, 59, and 66 of switch units 54 and 55 thus connect the grids and cathodes of cach tube in parallel with transformer I9 in accordance with the setting of the swingers. In this manner, the characteristics of each'of the twenty-one tubes within the capacity of the system may be compared with that of every other tube connected to the respective contacts, and a rapid comparison can thus be made.

The screen grids of each of the tubes 68, 69, and 16 and all other tubes being tested are connected over a common conductor 86 through a screen voltage divider 81 and condenser 69 to the ground conductor 99. A plate voltage resistor 9i and filter-condenser 92 are also provided. The cathode heaters may be connected to an energy source at terminals 94, while the anode voltage supply is connected at terminal 95.

To illustrate the method of making a test, a pair of tubes, such as 68 and 69, may be connected for test by throwing swingers 25, 32, and 64 to their second contacts, and swingers 24, 3I, and 62 to their third contacts. Switch 1 is atraveo `lnow thrown r-to the lett, which is the dynamic test position for'the 'tubes 68 and `t9. 'The clamp circuit, including tube v35, is now .connected in lthe circuit 'and Vthe grid voltage Virripressed on the tubes 68 and 69 is 4all negative. This 'is ac- V'eomplished 'by `the clamp circuit in the manner `shown in Fig. 12, wherein the alternating current output fof the oscillator 5 is represented by 'the V'alternating.current sine wave 'It having a :zero potential axis and which charges condenser i222 to substantially the amplitude of wave 110|) during the iirst positive half cycle of vthe wave.A However, since the tube shunts the output of -transformer I9 in one direction only, the lactual wave impressed on the input of the tubes to be tested is as shown by curve I-Il'I, wherein Asubstantially all of the wave is below the Zero axis of the wave l, there being only A'a very smal-l iiat portion at the crests of wave IDI 'due to the discharge of condenser 22 through resistor 34. However, 'for all practical purposes, the entire `wave Il is negative, and its peak value 'is essentially the same as that of wave |00. This type of wave is desired since the 'gain varying voltage impressed on the tubes in the 'normal variable gain circuit is all inthe negative direction, as shown in Fig. 3.

Continuing lthe method Y'of selecting a matched pair of tubes, when the sine wave l''I is impressed on'the tubes $8' and te, any imbalance between the characteristics will be indicated on meter 52; the tube 'being 'swung over substam tially its entire grid characteristic, las shown in -One vof the Kcriteria of the test is that 'the dierentifal current, as indicated at meter j52, be below '57 dbm, and if the indication at 52 shows a higher level, then either switch Yunits '2-6 or 2'I should be turned to select another tube. However, before another tube is selected for comparison with either 63 or Se, the balancing v'slider 91 is adjusted to obtain the null reading at `meter 52. It is this null reading which must be below -57 dbm.

Assuming now that a pair of tubes comes with- 'in the -5'7 dbm limit, then 'switch l 'is thrown' |00, shown in Fig. '2, but of reduced amplitude.

This wave will operate the tubes over a short V portion of the characteristic at a point of Zero gain reduction such as -3 volts and determine the correspondence 'of the two characteristics at this point. As shown in Fig. 3, assume one 'tube has a characteristic as Shown by the solid line IDS, and the other tube has a characteristic such as shown by the dot-anddash line I c5. Since these characteristics are nearly parallel over the majority of their lengths and over which voltage wave lill swings `the grids of the tubes, 'the dierential output of the dynamic test may well be below v-57 dbm. However, it `is noted that .the curves |65 and |08 depart from parallelism around the -3 volt grid bias point, and thus, the static test will indicate this condition. To obtain the minimum variation during this test, the potentiometer 91 is again adjusted to provide the minimum reading at meter 52. Now, if the variation between the upper portions of curve HB5 and curve 406 is not. too .gr-eat, this adjustment may bring the output level below -83 dbm, which has been set up as the limiting value for this test. If the Aoutput is Ll il 6 higher than '83 ldbm, .a new pair of tubes 'should :be tested.

Now, .if the pair of tubes .is satisfactory upy to this point,"y the :next operation is 'to `throw the switch?! again to .its :left-hand position. for a dynamic test reading, and without :any further adjustment vci the potentiometer 91, the `output .level then should be below 47 dbm. If this readingzis above this value, then a .new pair Aof `tubes should-be :connected into the test circuit. .A pair of tubes do not have to yhave identical characteristics throughout their entire range "to operate satisfactorily in a variable `gain circuit. That ;is,.a tube having a characteristic such as shown vby curve Il will operate satisfactorily with a tube having a characteristic such as shown by curve i525, since potentiometer 97, used in a'ccmmercial circuit, can easily match such tubes.. Furthermore, it will be noted that either the :dynamic or Vstatic test `.alone vwill not provide `sucient .information to enable the' selec tionfof `satisfactory tube pairs, since, as A:pointed out, a fgood pair ci tubes by the dynamic test may .not 'be satisfactory und-er the static test and vice versa. The static test could be satisfactory .if only small grid bias variations were to be applied in actual practice,-but the .larger negative swings are actually encountered. With both tests, however, `ai 'great `many pairs `of matched tubes may be quickly selected. for -use in commercial operating variable gain :amplifiers yor Vany push-pull yamplifier where balance is desirable.

Referring now to Fig. 4, tubes III'I and lll are shown connected in `a variable Ygain .ampli-der vof an operating circuit having a standard balancing network H3, output :amplifier ile, and a volume indicator meter I I5. Monitoring loudspeakers may also be connected into the circuit. The controlV rectier Il may be an independ ent. source of voltage .gain control, or may be the 4feedbacirfrom the outputof the variable .gain amplier, as disclosed in the above .mentioned Sing-er patent. The balance lcontrol potentiometer is shown `at II'F.

The .added portion to the standard circuit s shown in the heavy lines, a resistor ld simulating the internal impedance of the control rectifier .I |6 to maintain the `charging rate of a condenser I2I the same as in normal operation, resistor |22 being .the regular release resistor for the condenser. |.2|. vA source of direct current is indicated as a battery |24, the battery having several taps which would provide, for instance, three, six, and nine volts, depending upon the amount of .gain reduction desired. A swinger 125 selects the desired value of negative voltage to be applied to the grids in parallel and the momentary closing of a switch |26 applies the voltage.

This adjunct .permits of the adjusting vof :balancing network |-I3 and provides a quick check of the tubes in the field in the event one tube of the pair has yshifted or changed its characteristic .between the period of thetest 4in the `circuit of .Fig` 1 and their actual insertion in an operating circuit. By adjustment of switch |25 to the Vf current of substantially constant frequency, 'a

plurality of plural section switches adapted to connect and disconnect the inputs and outputs of a plurality of tubes to be matched one against the other, a push-pull circuit, a current measuring device for indicating the current in said push-pull circuit, certain sections of said switches connecting in parallel, the inputs of a pair of said tubes to said constant frequency source and certain other sections connecting the outputs of said pair of tubes to said push-pull circuit, and a clamp circuit for shifting the voltage output of said constant frequency source to a negative value.

2. A tube testing system in accordance with claim 1, in which additional switch means are provided for connecting and disconnecting said clamp circuit to said constant frequency source, and an attenuator is provided, said switch means connecting said attenuator into said system when said clamp circuit is eliminated from said system.

3. A tube testing system in accordance with claim 1, in which certain sections of said switches are ganged together, certain of said sections shifting the output of said constant frequency source between different tubes and other sections shifting the output between said tubes and said push-pull circuit.

4. A tube testing system in accordance with claim 1, in which said clamp circuit includes a diode vacuum tube, a series condenser, and a shunt resistor.

5. A testing system for obtaining vacuum tubes with similar characteristics comprising a source of alternating current having a certain frequency, a clamp circuit for shifting said alternating current to a wave having substantially all negative values, switching means for switching said clamp circuit in and out of said testing system, a parallel input circuit from said clamp circuit to the inputs of a pair of tubes to be compared, a push-pull output circuit for said tubes, and means for reading the diierential between the currents transmitted by said tubes, one position of said switching means providing a simultaneous grid variation for both of said tubes extending substantially over the entire negative characteristic of said tubes, and another position of said switching means providing a grid variation extending over a small limited negative portion of said characteristics of said tubes.

6. A system in accordance with claim 5, in which switching means are provided for connecting into said test system one or more of a plurality of vacuum tubes.

7. A system in accordance with claim 5, in which an attenuator is provided in said test system, said attenuator being introduced in said system when said clamp circuit is eliminated from said svstem.

8. The method of selecting a pair of tubes having substantially identical characteristics from among a plurality of tubes comprising simultaneously swinging the grids of a pair of said tubes over substantially the entire negative range of the characteristics of said tubes, measuring the differential current between the outputs of said tubes, said differential current having a predetermined maximum critical value, then simultaneously swinging said grids over a short portion of their characteristics, and measuring the differential current between the outputs of said tubes, said differential current having another predetermined maximum critical value.

9. The method in accordance with claim 8, in which the grids of said tubes are again simultane- 8 ously swung over substantiallytheir entire negative range, and again determining the differential current between the outputs of said tubes, said differential current having a third predetermined maximum critical value.

10. The method of detecting unbalance between two tubes adapted to operate in a push-pull circuit comprising simultaneously varying the negative potential on said tubes over substantially the entire negative portions of their characteristics, measuring the diierence between the output currents of said tubes, then simultaneously varying the negative potential on said tubes over a short portion of their characteristics near their Zero grid voltage point, again measuring the diierence between the output currents of said tubes, again simultaneously varying the negative potential on said tubes over substantially the entire negative portions of their characteristics, and again measuring the difference between the output currents of said tubes.

11. The method in accordance with claim 10, in which each current measurement must be below a certain predetermined value to provide a satisfactory operative pair of tubes, said last mentioned measurement having a higher value than said first mentioned measurement.

12. A tube testing circuit comprising a source of test voltage for a pair of tubes to be tested for similarity of their characteristics, a switch for connecting the inputs of said tubes to said source of voltage, a push-pull output circuit for said tubes, a current meter for measuring the difference in current output from said tubes, and connected to said push-pull circuit, means connected between said voltage source and the inputs to said tubes for shifting the input voltage to said tubes to negative values, and an attenuator connectable to said switch, said switch disconnecting said voltage shifting means and connecting said attenuator in the inputs to said tubes. y

13. A tube testing circuit in accordance with claim 12, in which a plurality of triple-sectioned switches are provided, a pair of said sections connecting the inputs of each of a plurality of tubes in parallel to said first mentioned switch, and the remaining sections of said switches connecting the outputs of a pair of said plurality of tubes to said push-pull output circuit.

14. A tube testing circuit in accordance with claim 12, in which said voltage shifting means is a diode connected in shunt across the inputs to said tubes.

15. A tube testing circuit comprising a source of test voltage, means adapted to connect the inputs of a plurality of tubes to be tested in parallel to said voltage source, a push-pull output circuit adapted to be connected to the outputs of a pair of said tubes, and a pair of triple-sectioned gang switches, the inputs of each of said tubes being connected to contacts on two sections of both of said gang switches and the outputs of all of said tubes being connected to contacts on the remaining sections of said gang switches.

JOHN J. DE MUTH.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,946,092 Miessner Feb. 6, 1934 1,948,303 Lavoie Feb. 20, 1934 2,385,186 Bayless Sept. 18, 1945 

